Edexcel Jun 2014 (R) Paper 5 Q21

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21 Chromium is a typical transition metal: it forms complexes, coloured compounds and exists in a range of stable oxidation states. Chromium and some of its compounds also show catalytic properties. (a) Define the term transition metal. (1)(b) The diagram below summarises some reactions of chromium compounds. reagent C dissolved in water add NaOH(aq) yellow solution W (NH4)2Cr2O7(s) heat Cr2O3(s) colourless organic liquid A and H2SO4(aq) a few drops of NaOH(aq) Cr3+(aq) element B and H2SO4(aq) blue solution Z shake mixture green precipitate X excess NaOH(aq) green solution Y (i) Identify, by name (including the oxidation state where appropriate) or formula, the species containing chromium in the sequence. (4) WXYZ*P42989rA01528* 15 Turn over<br /> (ii) Identify, by name or formula, suitable reagents for the sequence. (3) ABC(iii) Write the ionic equation for the reaction between (NH form the yellow solution. State symbols are not required. 4)2Cr2O7 and NaOH to (iv) When (NH4)2Cr2O7 is heated, steam and nitrogen are formed as well as Cr2O3. Write the equation for this reaction. State symbols are not required. Explain why this is a redox reaction, stating any changes in oxidation numbers that occur. (1) (3) Equation Explanation(v) Explain how shaking solution Z re-forms Cr3+(aq). (1)16 *P42989rA01628*<br /> (c) If excess aqueous ammonia is added to Cr3+(aq), the ammonia acts as a ligand and the resulting green solution contains a chromium species which is different from the one found in Y. (i) Explain the term ligand. (2)(ii) Write an equation for the reaction that occurs, showing all the ligands involved for both the chromium species in the reaction. State symbols are not required. (2) (Total for Question 21 = 17 marks) *P42989rA01728* 17 Turn over<br />

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